Carboxylation of benzoic acid



United States Patent 3,038,006 CARBOXYLATION 0F BENZOIC ACID Robert F. Ruthrutl, 18530 Klimm Ave., Homewood, 111. No Drawing. Filed Apr. 1, 1959, Ser. No. 803,358 7 Claims. (Cl. 260-415) This invention relates to the carboxylation of aromatic carboxylic acids. More particularly, this invention relates to the conversion of benzoic acid to benzene dicarboxylic acids. In one particular aspect thereof, this in- 10 vention relates to the carboxylation of benzoic acid to form terephthalic acid.

It is well known that on heating an alkali metal benzoate to incipient decomposition temperature some disproportionation occurs with the production of benzene dicarboxylic acids (as the alkali metal salts), specifically an alkali metal salt of terephthalic acid, together with an equivalent amount of benzene. Also it is known that on heating a mixture of an alkali metal benzoate and an alkali metal formate some carboxylation occurs with the formation of benzene dicarboxylic acids (as metal salts), specifically an alkali metal salt of terephthalic acid. However, in either reaction the yield of alkali metal salts of benzene dicarboxylic acids is so very small that both reactions are of pedagogical interest only.

More recently, it has been suggested that benzene dicarboxylic acids (as alkali metal salts), specifically an alkali metal salt of terephthalic acid, be produced by heating an alkali metal benzoate to a high temperature in the presence of ahigh partial pressure of carbon dioxide. While this procedure results in a higher yield of benzene dicarboxylic acids (as alkali metal salts), specifically an alkali metal salt of terephthalic acid, than the two reactions described in the previous paragraph, nevertheless here too the yield of the desired product is quite low and generally unsatisfactory. To achieve an even moderately rapid rate of reaction a high operating temperature must be employed, 350 C. of higher, usually 400 to 450 C. and even so reaction periods of six hours or more are required. These high temperatures and long reaction times result in the decarboxylation' of a considerable portion of the alkali metal benzoate charge with the production of benzene but at the same time a considerable portion of the alkali metal benzoate charged remains unconverted and may be recovered at the conclusion of the reaction. Obviously, both of these behavior patterns militate against high yields of the desired product of the reaction. Also, in order to favor the desired carboxylation reaction, the reaction vessel is charged with carbon dioxide at a high pressure ranging from about 85 atmospheres to 170 atmospheres. These high'v operating pressures, especially when coupled with the rather high reaction temperatures, employed, require strong and expensive autoclaves and auxiliary equipment.

Because of the severe operating conditions required and the generally unsatisfactory yields of the desired alkali metal salts of terephthalic acid, as far as the present ap plicant is aware this synthetic procedure has never been employed commercially.

I have found that alkali metal alkyl carbonates are highly efiicient carboxylat-ing. agents. Through use of alkali metal alkyl carbonates it is possible to convert an alkali metal benzoate to the alkali metal salt of a henzene dicarboxylic acid.

A principal object of this invention is to provide an improved method for the carboxylation of aromatic carboxylic acids.

Another object of this invention is to provide an improved method for the conversion of benzoic acid to benzene dicarboxylic acids.

A further object of this invention is to provide an im- "ice proved method for the carboxylation of benzoic acid to terephthalic acid.

Additional objects of this invention will appear as the description thereof proceeds.

Alkali metal alkyl carbonates which serve as the carboxylating agents of this invention are readily prepared by the action of carbon dioxide on an alkali metal alkoxide. As an example of such a procedure, a solution containing about 16.5% by weight sodium ethylate is prepared by the gradual addition of one gram atom metallic sodium to half a liter of absolute ethanol. This solution is then treated with at least the stoichiometric quantity of carbon dioxide which results in the precipitation of the desired sodium ethyl carbonate. 0n the laboratory scale the conversion of the sodium ethylate to sodium ethyl carbonate is readily and conveniently accomplished by the gradual addition of an excess of finely divided solid carbon dioxide to the solution of sodium ethylate in absolute ethanol. Alkali metal alkyl carbonates are practically insoluble in alcohols and accordingly, as previously indicated, this preparative procedure gives rise to a suspension of the alkali metal alkyl carbonate in the corresponding alcohol. This suspension may be used directly in the carboxylation reaction of this invention, it not being necessary to separate the alkali metal alkyl carbonate from the alcohol as a distinct step of the preparative procedure.

If desired, alkali metal alkyl carbonates may be made in the dry" way. Thus, solid sodium alkoxides may be prepared by the interaction of an alcohol vapor with molten sodium following which the sodium alkoxide, in finely divided form, is allowed to interact with gaseous carbon dioxide to form the solid sodium alkyl carbonate which may be used in solid form in the react-ion of this invention or may be suspended in an inert solvent (e.g.,

A reaction vessel provided with a short distillation column is charged with a slurry of one mole sodium ethyl carbonate in ethyl alcohol, the solids content of said slurry being 16.5% by weight. One mole sodium benzoate is then added and the resulting mixture is slowly heated to a temperature of about C., the free ethyl alcohol being recovered through the column. When this free alcohol has been eliminated, the exit from the reactor through the column is closed off and the dry residue in the reactor is gradually brought to a temperature of 300 C. over a period of two to three hours. During this time one mole of ethyl alcohol is theoretically produced to which must he added any free alcohol that may have escaped elimination during the initial distillation operation. Also, due to side reactions, a small amount of carbon dioxide may be produced from the reacting solids during this second heating period. If desired, the reactor may be vented as necessary during this second heating operation to maintain the pressure therein at some convenient level, say ten atmospheres gage.

After the reaction mixture has been brought to a temperature of 300 C. as described, any residual pressure in the reactor is vented to atmosphere and the reactor and its contents are cooled to below 100 C. Suflicient hot water is added to the reactor to dissolve all of the solids therein with the exception of a small amount of dark colored decomposition products usually produced and the resulting solution is filtered. The filtrate is made strongly acid (Congo red paper) with 2 N hydrochloric acid and then cooled. The precipitated acids are sepa- 3 rated by filtration and washed with a small amount of cold water.

The resulting mixture of acids was separated as completely as possible by fractional crystallization. 'Ihe separations were based on the fact that phthalic acid is very soluble in hot water while benzoic acid and terephthalic acid are only slightly soluble in hot water and further on the fact that terephthalic acid is essentially insoluble in cold water while'benzoic acid and phthalic acid are very slightly soluble therein.

A little less than 0.5 mole phthalic acid was recovered together with 0.35 mole unchanged benzoic acid. Only traces of terephthalic acid and isophthalic acid were found.

Example 2.

The general procedure described in Example 1 was repeated with the exception that in the present example one mole potassium benzoate was substituted for the mole of sodium benzoate employed in Example 1.

On processing the reaction product of the present example, there was obtained 0.65 mole mixed benzene dicarboxylic acids together with just under 0.25 mole unchanged benzoic acid. The 0.65 mole mixed benzene dicarboxylic acids comprised approximately 0.35 mole 'phthalic acid and 0.30 mole terephthalic acid. Only a trace of isophthalic acid could be isolated.

Example 3 The general procedure described in Example 1 was repeated with the exception that in the present example one mole potassium benzoate was substituted for the mole of sodium benzoate employed in Example 1 and one mole potassium ethyl carbonate was substituted for the mole of sodium ethyl carbonate employed in Example ,1.

The reaction product of the present example yielded r 0.70 mole mixed benzene dicarboxylic acids together with 0.20 mole unchanged benzoic acid. The 0.70 mole mixed benzene dicarboxylic acids comprised approximately 0.45 mole terephthalic acid and 0.25 mole phthalic acid with only a trace of isophthalic acid.

The carboxylation reaction of this invention may be conducted in the temperature range 200 to 350 C.,

conversion. All points of view considered, a reaction temperature in the approximate range 275 to 325' C. appears best and accordingly is preferred.

While this invent-ion has been described in connection with the carboxylation of alkali metal benzoates it is not limited thereto. By the process of this invention it is possible to carboxylate the alkali metal salts of aromatic carboxylic acids in general. Thus, when subjected to the procedure of this invention, an alkali metal beta naphthoate is readily converted into the alkali metal salt of 2,6-naphthalene dicarboxylic acid.

Be it remembered that while this invention has been described in connection with specific details and specific embodiments thereof, these details and embodiments are illustrative only and are not to be considered limitations on the spirit and scope of said invention except in so far as these may he incorporated in the appended claims.

I claim:

1. In the conversion of benzoic acid to a benzene dicarboxylic acid, the step of heating in the temperature range 200 to 350 C. a mixture of an alkali metal benzoate and an alkali metal alkyl carbonate.

2. In the conversion of benzoic acid to a benzene dicarboxylic acid, the step of heating in the temperature range 200 to 350 C. a mixture of sodium benzoate and a sodium alkyl carbonate. 3. In the conversion of benzoic acid to a benzene dicarboxylic acid, the step of heating in the temperature range 200 to 350 C. a mixture of sodium benzoate and sodium ethyl carbonate.

4. In the conversion of benzoic acid to a benzene dicarboxylic acid, the step of heating in the temperature range 200 to 350 C. a mixture of potassium benzoate and a sodium alkyl carbonate. 3

5. In the conversion of benzoic acid to a benzene dicarboxylic acid, the step of heating in the temperature range 200 to 350 C. a mixture of potassium benzoate and sodium ethyl carbonate.

6. In the conversion of benzoic acid to a benzene dicarboxylic acid, the step of heating in the temperature range 200 to .350 C. a mixture of potassium benzoate and a potassium alkyl carbonate.

7. In the conversion of benzoic acid to a benzene dicarboxylic acid, the step of heating in the temperature range 200 to 350 C. a mixture of potassium benzoate and potassium ethyl carbonate.

References Cited in the file of this patent UNITED STATES PATENTS 2,823,230 Raecke Feb. 11, 1958 2,906,774 Raecke et al. Sept. 29, 1959 

1. IN THE CONVERSION OF BENZOIC A CID TO A BENZENE DICARBOXYLIC ACID, THE STEP OF HEATING IN THE TEMPERATURE RANGE 200* TO 350* C. A MIXTURE OF AN ALKALI METAL BENZOATE AND AN ALKALI METAL ALKYL CARBONATE. 